Anodic methoxylation of 2,5-dimethyl- and tetramethyl-thiophene. Formation and transformation of 2,5-dimethoxy adducts

Abstract

The electrooxidation of 2,5-dimethyl- and tetramethyl-thiophene in methanol containing sodium methoxide produced isomeric mixtures (in either case 40%cis, 60%trans) of the corresponding 2,5-dimethoxy adducts each in 30%yield, together with side-chain oxidation products. During GLC analysis a portion of the 2,5-dimethoxy adduct of 2,5-dimethylthiophene loses a methanol molecule to give 2,5-dihydro-2-methoxy-2-methyl-5-methylenethiophene. By contrast, the 2,5-dimethoxy adduct of the tetramethyl compound remained intact on GLC. In CDCI₃ solution in an NMR tube, each isomer of the 2,5-dimethoxy adducts was transformed into the corresponding 2-(methoxymethyl)thiophene. When sodium acetate was used as electrolyte, the products were the corresponding 2-(methoxy-methyl)thiophenes plus thiophene-2-carbaldehydes. In order to synthesize 2-acetoxymethyl-5- methylthiophene as a reference compound, anodic acetoxylation of 2,5-dimethylthiophene was also examined.